This invention relates to transportation systems in which vehicles travel through tunnels formed by boring in the earth or by cutting trenches in the earth and subsequently covering them, and more especially, though not exclusively, to mass-transmit and other railroad systems; it also relates to electric cables that are designed specifically for use therein.
Large quantities of electric cable are installed in such systems for such purposes as signalling, trackside communication, track-to-train communication (radiating cables), provision of traction current, and auxiliary power circuits for the operation of route switches and other trackside equipment, station lighting, etc. For some of these purposes incombustible cables with compacted mineral insulation could be used, but for most the use of polymeric material in the insulation or jacket or both is a practical necessity. There is always some risk that polymeric materials, even though formulated for enhanced flame retardance, will burn if pre-heated to a high temperature by an external source, such as an external fire, with the risks that: (i) in certain circumstances fire may be transmitted along cable runs through the tunnels to other parts of the system; (ii) dense smoke may be generated; and (iii) since PVC and/or other halogen-containing materials are commonly present toxic and corrosive fumes (such as hydrogen chloride gas and/or hydrochloric acid droplets) may be produced. Obviously these risks must be minimised in the interest of the safety and comfort of passengers and the reliable and uninterrupted operation of the system.
Each of these risks, considered separately, can be avoided or at least contained by conventional formulation techniques, but the requirements are in conflict because most flame-retardant additives that are effective in halogen-free polymers themselves contain halogen and so contribute seriously to the hazard of toxic and corrosive fumes if the cables are burned, and there is a tendency for low-flammibility polymer compositions to smoulder and generate dense smoke when they do burn. Furthermore, improvements in characteristics that reduce fire hazards almost always do so at the expense of the mechanical and/or electrical properties of the composition.
It is an object of the invention to provide a cable for use in transportation systems that is halogen free, has adequate mechanical and electrical properties and combines a high oxygen index (low flammability) with the characteristic of producing, when burned under typical fire conditions, little visible smoke and no major quantity of toxic or corrosive fumes. The tunnels of a transportation system comprise a unique environment for the installation of electric cables, since they are subject neither to the chemical and mechanical effects of soil as are cables buried in the earth, nor to the effects of temperature extremes, sunlight, weather, abrasion or impact generally encountered above ground level. Accordingly they call for cables differently-optimised compared with those for other environments.
Joyce A. North et al. in U.S. Pat. No. 3,922,442 issued Nov. 25, 1975, have described uniinsulation compounds for electric cables based on copolymers of ethylene with vinyl acetate and containing large amounts of hydrated alumina and minor but essential amounts of silane. The experimental basis for North et al.'s teaching appears to be restricted to the use of plastic (non-rubbery) copolymers with either 17 or 28% vinyl acetate and filler levels up to 150 phr (parts per hundred of polymer). Within this range the applicants have no reason to doubt the teaching of North et al.; we have however worked with rubbery polymers with substantially more than 28% vinyl acetate and with higher filler loadings, and have found the teaching of the patent completely inaccurate and misleading in its references to these ranges.
Firstly, we have found that the use of the very expensive silane ingredient is not merely inessential but positively harmful when the polymer contains over 28% vinyl acetate; it has no significant effect on the compatibility of the filler with the polymer or on fire performance, and a marginal improvement in tensile strength of the blend is outweighed by a drastic loss of elongation.
Secondly, we have found that in these rubbery, high vinyl acetate copolymers the particle size of the hydrated alumina filler is critical to obtaining a high loading and a processable mixture, in direct contradiction of the teaching of North et al.
Thirdly, we have found that a loading of 400 phr or more of filler, as proposed by North et al is impossible to achieve by a considerable margin, and that a loading significantly over 150 phr is possible only when the vinyl acetate content of the polymer is well over 28%.
Furthermore we have made two surprising discoveries. Firstly, that by using silane-free high-vinyl acetate copolymer compositions with a controlled high loading of hydrated alumina it is possible to achieve, in a halogen-free low-smoke material with adequate physical properties for transportation cables, an increase in oxygen index that is very large compared with the marginal improvement obtained by North et al. (note that the highest oxygen index achieved by them for a composition of their invention was 28, which failed even to equal their own silane-free comparison example 6 when vulcanised (see tables C and E); the non-standard test with coated wires cannot be compared with standard tests on the composition itself and is not relevant to our purposes). Secondly, we have found that when our material is used for the outer, protective layers of a transportation cable, a significant and surprising improvement in smoke emission characteristics and in some cases in fire survival as well can be obtained by interposing a very thin aromatic polyimide tape layer between two layers of the filled copolymer composition.